But I can be. I'm one of the Black Mages running around on here.
First of all you need a goto spell. You're specialty spell. Ideally it's not the highest one you can cast so you can apply some Metamagics with it. SpC's Orb of X is a goto favorite, 15d6 cap, no SR check, even has a secondary minor debuff. However you can use quite a few other things, including oddballs Laim's finger Darts or Shivering Touch to nuke their Ability Score(s) away.
Pick one and abuse the crap out of Metamagics. The mailman link someone already posted uses a Sorcerer casting Greater Arcane Fusion(arcanefusion[magic missile, magic missile] moar magic missile) with all those Twinned/Repeat Meta magic it takes like ten rounds to resolve. And that's one style. I've always aimed more towards elemental based with crowd control. Like if you use Snowcast to tack on the [Cold] Descriptor you can improve it's Save DC by +6 and gain up to +3 damage
per die. And if you go back and read Empower Spell per PHB (not SRD), that 1d6+3 is [4~9+50%] not [1d6+50%+3] for a rather nice increase in damage.
But anyway. Class wise look into Sorcerer over Wizard. Sure Wizard gets spells sooner but unless they are a Focused Abyssal Specialist they cast less per day and you need a lot to nuke stuff. Plus the Sorcerer side gets Wings of Furry (4th level uncapped area burst that stuns), Arcane Fusion (cast two spells at once), Arcane Fury (moar doublecast), access to Practice Metamagic (reduce metamagic costs by 1), and an easy trade in for Dragonic Heritage which helps set up the Draconic Aura that gives up +4 to your Save DCs.
Class wise, Shipwreck's Stormcaster takes a single CL hit to augment, if done correctly, Save or Stun onto all your spells as well as a little extra Sonic damage which is pretty damn awesome. Prestige Bard trades a CL as well but the level dip into it gives access to some nice Charisma boosts and party buffs, including the possibility of Dragonfire Inspiration with the Summon Monster spells. Recaster is the last one I'd suggest out of the PrCs that lose CL, it trades one away for 3/Quicken and 5/day Empower or Maximize and you even net the ability to reshape the area on Spells. Full caster PRCs, Incantatrix is made of win if you can pick it up. Warmage out of Dragonlance is
really nice with it's ConMod/Day +3 per die boost and Cha to AC for all nearby allies. Other than those you can shoot for anything that goes with your focus. Like you wanting to hit people with Negative Energy? Hunt around for some nice Necromancer flavored PrCs.
The biggest draw back in being a nuker is TPKs from your poor aim. There is a few items that prevent your spells from frying your party members through. Can't recall any by name through...
OH and don't forget to hit up
Class Boosters. I actually have a how to maximize Charisma section in there under Sorcerer, how to nuke people with four or five elements at once under Cold Casting, and even a list on how to obtain additional Feats in case you wanted to buy Empower Spell for 5,000gp instead of using a Feat Slot.